The paramyxoviruses infect both humans and animals. The group includes well-known human pathogens such as parainfluenza viruses of which several types are known, measles virus, mumps virus,and respiratory syncytial virus. Among the animal-pathogenic paramyxoviruses are canine distemper virus, rinderpest virus, and Newcastle disease virus. The paramyxoviruses are single-stranded RNA viruses. The virus particles have an outer membrane with short projecting spikes, and an inner nucleocapsid. The spikes include two glycoproteins of relevance, a hemagglutinin with neuraminidase activity (HN) and a cell fusion-inducing protein (F).
Pathogenesis and virus spread through the infected host are both facilitated by virus-mediated cell fusion. Recent work has demonstrated that both HN and F are required for cell fusion to occur [Hu, X. et al. (1992) J. Virol. 66:1528-1534; Morrison et al. (1991), J. Virol. 65:813-822; Katzir, Z. et al. (1989) Biochemistry 28:6400-6405; Ebata, S. N. et al. (1991) Virology 183:437-441]. The genes encoding HN and F, respectively, have been cloned from several virus types, including Sendai (SN), simian virus 5 (SV5) parainfluenza type 1 (PI1), parainfluenza type 2 (PI2), and parainfluenza type 3 (PI3). The respective proteins have been purified and various polyclonal and monoclonal antibodies against them have been obtained. Substantial sequence differences exist between the respective HN and F proteins of SN, PI1, PI2, and PI3, although the greatest sequence similarities exist between SN and PI1 [Matsuoka, Y. et al. (1990) Virus Res. 16:107-114; Blumberg, B. et al. (1985) Cell 41:269-278; Elango, N. et al. (1986) J. Virol. 57:481-489; Merson, J. R. et al. (1988) Virology 167:97-105; Millar, N. et al. (1986) J. Gen. Virol. 67:1917-1927]. HN and F can be expressed either individually or together in HeLa-T4 cells using a vaccinia-T7 recombinant virus vector system [Fuerst, et al. (1986) Proc. Nat'l. Acad. Sci. USA 83:8122-8126].
Various experimental approaches have been taken in an attempt to further define the respective roles of HN and F in cell fusion. The possibility of direct association of HN and F has been considered, however, the evidence to date has been negative. Initial studies by Markwell, M. A. K. and Fox, C. F. (1980) J. Virol. 33:152-166 demonstrated that HN of Sendai and Newcastle disease viruses exists as a disulfide linked homodimer. The authors also studied spatial relationships of viral proteins by observing the effects of chemical cross-linking reagents on proteins of the mature virus. Interactions up to 1.1 nm distance were expected to be detectable. The NP (nucleoprotein) and M (membrane) proteins were cross-linked by the reagents. Also the F protein could be cross-linked to itself, suggesting that F exists in the native virus as noncovalent homo-oligomers. No close association between HN and F was observed in the cross-linking studies. Sechoy, O. et al. (1987) J. Biol. Chem. 262:11519-11523 extended the previous study with respect to F, using a system of purified F in reconstituted lipid vesicles. The authors concluded that native F exists as a noncovalent association of homo-tetramers, each consisting of two peptides, F.sub.1 and F.sub.2, linked by a disulfide bond. (F.sub.1 and F.sub.2 are formed by posttranslational cleavage of a precursor, F.sub.0 [Scheid, A. and Choppin, P. W. (1974) Virology 57:475-490; Homma, M. and Ohuchi, M. (1973) J. Virol. 12:1457-1465]. Citovsky, V. et al. (1986) J. Biol. Chem. 261:2235-2239 studied HN and F in reconstituted lipid vesicles by circular dichroism. HN and F were purified from detergent-extracted virus particles. The circular dichroism spectrum of vesicles reconstituted with both HN and F was shifted with respect to the spectra of vesicles reconstituted with HN or F alone. The authors inferred a changed conformation of the glycoproteins of the HN-F vesicles, possibly as the result of action of HN upon F. Nakanishi, M. et al. (1982) Exp. Cell Res. 142:95-101 also studied purified HN and F in reconstituted vesicles, using diphtheria toxin A fragment to confer cytotoxicity on the vesicles and varying proportions of HN and F. Optimum cytotoxicity of the vesicles was observed when the ratio of F to HN was two. Roux, L. (1990) Virology 175:161-166 studied the interaction of HN and F with the immunoglobulin heavy chain binding protein (BiP) by measuring the amount of HN or F precipitated by a monoclonal antibody to BiP. BiP belongs to the heat shock family. It is proposed to function as a "chaperon" protein to aid in maintaining proper folding of other nascent proteins during maturation. Anti-BiP antibody was found to precipitate HN in infected cells, along with BiP, however, only one-fifth as much F was coprecipitated as HN. Katzir, Z. et al. (1989) Biochemistry 28:6400-6405 set out specifically to test whether F-HN complexes are formed in infected cell membranes. The authors measured fluorescence photobleaching recovery as a measure of each component's lateral mobility in the membrane. One component was immobilized with an antigen and the effect on mobility of the other, labeled with a fluorophore, was investigated. The authors did not find any effect of HN on lateral mobility of F, or vice-versa, suggesting that no HN-F complexes formed. For a general review, see Morrison T. and Portner, A. (1991) Structure, function and processing of the glycoproteins of Paramyxoviridae, In The Paramyxoviruses (D. W. Kingsbury ed.) Plenum Publishing Co., New York, pp. 347-382.